Project Summary/Abstract: Cyclophosphamide (CPP)-induced injury to bladder urothelium can lead to life-threatening health conditions including hemorrhagic cystitis and bladder cancer. The application?s broad long-term objectives are to identify mechanisms driving CPP bladder injury and thus therapeutic targets. CPP induces urothelial loss from apoptosis and necrosis within 48 hours, followed by proliferation and repair finishing by 14 days. While fibroblast growth factor receptor 2 (Fgfr2) activity blunts injury in other tissues, roles of Fgfr2 in CPP-induced urothelial injury are unclear. Preliminary data shows that Fgfr2: 1) dampens urothelial cell loss and 2) enhances urothelial repair after CPP injury. Regarding cell loss, injection of keratinocyte growth factor (KGF), a ligand for epithelial Fgfr2, 24 hours prior to CPP in wildtype mice blocks apoptotic urothelial cell loss. Akt, a transducer of Fgf signaling and known repressor of apoptosis, is upregulated in urothelium after KGF. Regarding regeneration, mice with conditional deletion of Fgfr2 in all bladder urothelial layers have defective urothelial repair after injury. At 3 days post-CPP, controls had urothelial hyperplasia, significant restoration of uroplakin (barrier protein) staining and major resolution of inflammation and hemorrhage, while mutant bladders had less hyperplasia, attenuated uroplakin staining, and ongoing hemorrhage and inflammation. While both mutants and controls had expansion of Keratin 14 (Krt14)-positive putative progenitor cells across basal layers 3 days after CPP, mutant Krt14+ cells were hypertrophic with enlarged nuclei suggesting a cell cycle defect. Cell cycle profiling and assays for DNA content suggest that mutant Krt14+ cells have aberrant endoreplication (DNA replication without completion of mitosis, leading to polyploidy). Fgfr2-mutant cells undergoing apparent endoreplication also had evidence of increased DNA damage/replication stress 3 days after injury. Given that Fgfr2 stimulates Erk that can suppress cell cycle entry and endoreplication, Erk and its readouts were assessed and both were reduced in mutants 3 days after CPP. Regeneration defects, including large (likely polyploid) Krt14+ cells, persisted 10 days after CPP. Together, the hypothesis is that Fgfr2 signaling ameliorates urothelial injury from CPP by suppressing apoptosis via Akt and promotes regeneration after CPP by repressing endoreplication of urothelium via Erk. To test this hypothesis, the following Aims are proposed: Aim 1: Investigate how stimulation of Fgfr2 signaling blocks CPP- induced bladder urothelial cell loss. Whole animal and cell-based assays will elucidate roles of Fgfr2/Akt to block CPP-induced apoptotic injury. Akt inhibitors will be used prior to KGF to show roles of Akt downstream of Fgfr2. Aim 2: Determine how Fgfr2 promotes regeneration of bladder urothelium after CPP-injury. Whole animal and cell-based assays will elucidate roles of Fgfr2/Erk to suppress endoreplication and drive regeneration, including genetic rescue with constitutively active Erk in mutants and chemical rescue to accelerate repair in controls. Aim 3: Identify the cell-specific roles of Fgfr2 signaling in the bladder urothelium after CPP injury. Fgfr2 will be deleted in specific urothelial layers to identify cell-specific actions after CPP-injury.